1. Field of the Invention
The invention relates to a flexible force sensor of coupling type and multi-directional recognitions, and more particularly to a flexible force sensor capable of sensing normal stresses and shear stresses.
2. Description of the Related Art
In general, the force sensors with normal and shear sensing are mostly bulk-shaped and therefore not applicable to detection on irregular surfaces. Furthermore, conventional force sensors are difficult to sense normal stresses, shear stresses, or both in the array-type detection of the large area.
U.S. Pat. No. 6,951,143 discloses a stress sensor of coupling type, wherein a square-based pyramid has a strain gauge on each side thereof. A first pair of strain gauges disposed along a first pair of respective axes that intersect and the other pair of strain gauges, generally orthogonal to first direction, disposed along a second pair of respective axes that intersect. The deformations of a pair of strain gauges on two opposite sides of the square-based pyramid are different when a shear stress, along X or Y directions, is applied to the sensor. One of the strain gauge is tensed, and the other is compressed. Therefore, the magnitude and direction of the shear stress can be calculated. The deformations of the four strain gauges are equal after a normal stress is applied to the sensor. The magnitude and direction of the normal stress can be also calculated.
U.S. Pat. No. RE37065 discloses a normal and shear force sensor using reflections of ultrasonic waves. The sensor includes a plurality of ultrasonic transducers and a hemispherical element. The transducers are oriented at an oblique angle to the plane and aimed at the hemispherical element. The hemispherical element is displaced as a lateral shear stress is applied to the hemispherical element. Thus, the transit paths and the transit times of the ultrasonic waves are different from two opposite sides of the transducer. The change of the transit time can be used to calculate the magnitude and direction of the shear stress. When a normal stress is applied to the hemispherical element, the transit paths of the ultrasonic waves on two opposite sides of the hemispherical element are equal. Thus, the reflection times of the ultrasonic waves are the same. Accordingly, the magnitude and direction of the normal stress can be calculated.